This invention relates generally to gas purification installations. More particularly, this invention relates to a method and an installation for removing granular or pulverulent solid residues from an installation for purifying gases, especially blast furnace gases.
Installations for purifying gases, especially blast furnace gases, are known, in which the granular or pulverulent solid pollutants are separated from the gaseous phase using dry separators such as, for example, dust bags, cyclones, sack filters and electrostatic filters. These solid residues are collected in hoppers installed directly below the dry separators.
These hoppers, which need to be emptied regularly, have hitherto discharged the solid residues freely, either directly into lorry containers or wagons, or simply onto a pile below the hoppers to be subsequently loaded by mechanical shovels onto wagons or lorries. The lorries then remove the solid residues to an intermediate storage area. It will be noted that the solid residues separated from the blast furnace gases mainly consist of iron and coke dust which, under certain conditions, can be advantageously recycled in an agglomeration installation or can be reinjected directly into the blast furnace.
The operation of removing the solid residues from the filter hoppers is, in the current state of the art, essentially a discontinuous handling operation which has major disadvantages. First, free tipping of pulverulent solid resides in an operation which generates a large amount of dust, which certainly leads to problems from the point of view of cleanliness of the workplace and protection of the environment. Next, pouring solid residues into free air also liberates toxic gases and vapors in an uncontrolled manner, and these are entrained by the solid residues out of the gas purification installation when the hoppers are discharged. These gases and vapors liberated uncontrollably definitely represent major safety problems. Finally, the solid residues must be loaded by mechanical shovels onto wagons or lorries which transport them to an intermediate storage area in order, as required, to be subjected to an additional handling operation before recycling. It is clear that this discontinuous handling of the solid residues is an unhygienic, polluting and expensive practice. Furthermore, the mode of removal described hereinabove has the major disadvantage of not profitably using the considerable heat energy still contained in the solid residues at the output of the gas purification installation in a recycling operation.
It might naturally be thought to use continuous transport systems for granular or pulverulent products which are known per se, especially open mechanical conveyors (for example conveyor belts), mechanical conveyors integrated in closed pipes (for example Archimedes screws) and pneumatic conveyors. A priori, the above mentioned systems seem however, to create more problems than they solve. Open conveyors in no way eliminate the problems of pollution, cleanliness and safety linked with the dust, gas and vapors liberated during handling of the solid residues. Mechanical conveyors integrated in closed pipes might, being leak-tight systems, solve the problem of the liberation of dust, gas and vapors but, for reasons of cost, it is nevertheless inconceivable to use such systems over larger distances. As regards pneumatic conveyors, they lack reliability, because they risk becoming blocked when the solid residues are moist, which leads to significant cleaning work before they can again be restarted. Now, in the case of blast furnace gases, it is necessary to provide, inside the gas purification installation, for certain operating regimes of the blast furnace, partial condensation of the water vapor contained in the gases. This naturally leads to moistening of the solid residues collected in the hopper of the filter. This moistening of the solid residues can also result from an operation of adjusting the temperature of the blast furnace gases upstream of the filter, which is carried out by an injection of water. It must also be borne in mind that any transport system using a closed mechanical conveyor or a pneumatic conveyor can present serious risks of explosion if the gases entrained by the solid residues include combustible gases.
Another factor to be taken into account is the abrasive properties of the blast furnace dust. This is, in fact, dust with a relatively coarse particle size (of the order of one millimeter), which is composed of particles of high hardness.
It is apparent that there is a great need to provide a reliable method of continuously removing these solid residues, which at the same time, controls and eliminates the risks of such an operation. The risks are safety, cleanliness, and avoidance of pollution. In addition, the efficiency of the operation should not be compromised when the solid residues being removed are moist. The economic benefits of such a method and device for the removal of granular or pulverulent solid residues from a gas purification installation, especially blast furnace gases is obvious.